A search is constantly being made for less expensive raw materials to be used in producing glass. Naturally occurring materials, such as basalt and some shales, can be used to produce glass. These naturally occurring materials, however, contain iron oxides. During melting, some of this iron oxide is reduced and a slag containing iron and iron compounds is produced. This slag material is denser than the molten glass and tends to accumulate in the bottom of the glass furnace enclosure. This slag containing iron and iron compounds tends to contaminate and rapidly degrade not only the molybdenum used for electrodes, but also the platinum used for heater strips, bushings and thermocouple probes. Should the level of the slag containing iron and iron compounds accumulate sufficiently to flow into the forehearth of the furnace, extensive damage can be done to the platinum bushings used in fiber forming and to fiberizers used in the glass wool making process. This invention discloses a method and apparatus for determining when the slag containing iron and iron compound reaches the level where corrective action must be taken.
Molten glass is electrically conductive but presents a high resistance to the flow of electrical current. This principle was used by Wadham et al., U.S. Pat. No. 1,961,893, to determine the level of molten glass in a forehearth by inserting electrodes at varying heights in the molten glass. If a given electrode was conducting electricity, then it was positioned in the molten glass. If the electrode was not conducting electricity, then it was positioned above the molten glass. Varrasso et al., U.S. Pat. No. 4,063,027, measured the resistance between electrodes in a glass melting furnace to determine if the electrodes had slumped, shifted or broken. Varrasso et al. were detecting when the resistance between the electrodes increased as an indication of problems with the electrode positions.
Slag containing iron or iron compounds is highly electrically conductive. When the slag contacts the electrical conductive elements, the resistance between the electrically conductive elements drops. This drop in resistance indicates that the slag has accumulated to the level of the conductive elements.